Distribution and Interference of Common Milkweed (Asclepias syriaca) in Nebraska

Infestations of common milkweed (Asclepias syriacaL.), in descending order of incidence in Nebraska, occurred on areas occupied by soybean [Glycine max(L.) Merr.], oats (Avena sativaL.), sorghum [Sorghum bicolor(L.) Moench.], railroad rights-of-way, fallow land, roadsides, corn (Zea maysL.), winter wheat (Triticum aestivumL.), pasture, and alfalfa (Medicago sativaL.). Average crop yield reductions, within common milkweed communities of 11100 to 45 200 plants/ha, ranged from 2 to 10% for corn, 4 to 29% for sorghum, and 12 to 19% for soybeans. Reductions in crop yields tended to increase with increasing common milkweed populations. Germination percentage of common milkweed and sorghum were not significantly affected by growing medium conductivities below 7 and 13 mmho/cm2, respectively. Growth media at pH 2 significantly reduced germination percentage, hypocotyl length, and radicle length of both common milkweed and sorghum as compared with pH 4 to 10. Undiluted aqueous extracts of common milkweed shoot and root material significantly reduced germination percentage, hypocotyl length, and radicle length of sorghum.